1. Field of the Invention
The present invention relates to an apparatus for knock detection in an internal combustion engine and, more particularly, to an apparatus for knock detection comprising one or more knock sensors for detection of oscillations produced during knocking combustion events in an internal combustion engine and means for analyzing the signals from the knock sensors, which includes at least one input, a multiplexer and an analog/digital converter and which converts the analog signals supplied by or from the knock sensor into digital signals.
2. Prior Art
Under certain operating conditions in an internal combustion engine knocking combustion can occur, which is to be prevented. Knock sensors are associated with each cylinder or certain predetermined cylinders of the internal combustion engine, which supply output signals, which have fluctuating or oscillating components characteristic of knock when knock is occurring. The evaluation of the signals supplied by the knock sensors is difficult, since background and/or interference noise is present besides the portion of the signal originating from knock. The noise depends on various operating conditions, for example on the rotation speed or the load on the internal combustion engine.
There have already been a number of proposals for overcoming these difficulties. These proposals usually involve analog signal processing methods. In these methods both the signals characteristic of knock and also the background signals are determined by means of analog signal processing.
The apparatus for detection of knock in an internal combustion engine, in which the signal processing occurs at least partially with the help of a digital signal analyzing device, is described for example in German Patent DE-PS 31 37 016. In this known apparatus for knock detection the signal supplied by the knock sensor, which includes the portion of the signal characteristic of knock and the background portion, is amplified, filtered, rectified and integrated with the help of an analog component or circuit. The integrated signal is compared with a background signal, which is obtained by suitable filtration from the integrated signal. For determination of the background portion of the signal the integrated signal is first converted with an analog to digital converter and then processed with the help of a digital filter. The comparison of the integrated values with the background signals obtained by the digital filtration allows the knock to be detected. Knock is recognized when the integrated signal values deviate in a known manner from the background signal.
The analog circuits are usually built as integrated circuits. These analog ICs are rather inflexible and do not always satisfy the growing operational requirements for signal analysis. Several filters are placed in the analog analysis ICs. The filter that provides the best knock detection must then be determined. If several resonance frequencies clearly stand out in the knock signal or a significant noise frequency is present, the knock signal is usually unsatisfactory for knock detection or noise and background can only be masked out in an unsatisfactory manner.
It is an object of the present invention to provide an improved apparatus for detection of knock in an internal combustion engine, which does not have the above-described deficiencies.
It is also an object of the present invention to provide a method of knock detection using the improved apparatus.
These objects and others, which will be made more apparent hereinafter, are attained in an apparatus for detecting knock in an internal combustion engine comprising one or more knock sensors for detection of oscillations produced by the knocking combustion and means for knock signal preparation, which include at least one input, a multiplexer and an analog/digital converter and which convert the analog output signals from or by the knock sensor or sensors into a digital output signal.
According to the invention the apparatus includes at least two symmetric knock sensor inputs for signals from respective knock sensors and digital signal processing circuit means, which analyzes a digitized signal from the knock sensors, includes means for determining whether or not knock is occurring from the digitized signal. The means for determining whether or not knock is occurring preferably includes at least one digital filter device.
The apparatus for detection of knock in an internal combustion engine has the advantage that several frequencies are analyzed and interference frequencies that are present in the signal that is obtained can be suppressed. This provides an advantageous project-specific flexibility, which leads to improved knock detection, which is better than that obtained currently with analog signal processing means. Furthermore a differential signal analysis of the connected knock sensors is possible, which has the advantage that the coupling of interference with the knock signals is additionally minimized.
The apparatus according to the invention has only one analog input circuit, which supplies the knock sensor signals and performs a first signal preparation. The actual signal analysis and knock detection occurs only in a purely digital circuit portion. This digital circuit portion includes signal processing means, for example a filter, which are easily adjustable to the requirements to be expected, which allow a project-specific flexibility and which leads to improved knock detection. The digital signal processing means advantageously permits an energy analysis or a peak analysis of the sensor signals.
Preferred embodiments of the apparatus for detection of knock in an internal combustion engine are described in the following. It is particularly advantageous that different embodiments of the digital signal processing means can be provided without great effort or expense. Advantageously the analog input-preparing part of the different apparatus according to the invention is identical for all alternative digital solutions according to the invention.
The following components are useful in preferred alternative embodiments for the digital signal processing means for energy analysis: a filter, a rectifier or squaring circuit and integrator connected in series or means for performing a discrete Fourier Transformation (DFT) or means for performing a rapid Fourier Transformation (FFT). All three alternatives permit analysis of several frequency ranges and suppression of interference frequencies that are present. One or more knock indicators for combustion are then produced from this analysis. They are then compared with threshold values in order to decide whether or not knock is occurring or not. In this comparison reference values for knock-free operation are also considered.